Python plugin

The plugin adds several aliases for useful Python commands.

To use it, add python to the plugins array in your zshrc file:

plugins=(... python)

Aliases

Command	Description
`py`	Runs `python3`. Only set if `py` is not installed.
`ipython`	Runs the appropriate `ipython` version according to the activated virtualenv
`pyfind`	Finds .py files recursively in the current directory
`pyclean [dirs]`	Deletes byte-code and cache files from a list of directories or the current one
`pygrep <text>`	Looks for `text` in `*.py` files in the current directory, recursively
`pyuserpaths`	Add user site-packages folders to `PYTHONPATH`, for Python 2 and 3
`pyserver`	Starts an HTTP server on the current directory (use `--directory` for a different one)

The plugin provides three utilities to manage Python 3.3+ venv virtual environments:

mkv [name]: Make a new virtual environment called name (else $PYTHON_VENV_NAME, else "venv") in the current directory.
vrun [name]: Activate the virtual environment called name (else $PYTHON_VENV_NAME, else "venv") in the current directory.
auto_vrun: Automatically activate the venv virtual environment when cd’ing into a directory containing venv/bin/activate, and automatically deactivate it when cd’ing into any other directory, including subdirectories.
- To enable, set PYTHON_AUTO_VRUN to anything. For example:
```
export PYTHON_AUTO_VRUN='true'
```
- To disable, either unset the environment variable:
```
unset PYTHON_AUTO_VRUN
```
  or entirely remove auto_vrun from the list of functions associated with the chpwd hook:
```
add-zsh-hook -d chpwd auto_vrun
```
  If you disable auto_vrun while a virtual environment is active, you'll need to manually deactivate.
- The default virtual environment name is "venv". To use a different name, set PYTHON_VENV_NAME. For example:
```
export PYTHON_VENV_NAME='.venv'
```